The present invention relates to an organic material for an electroluminescent device and an electroluminescent (EL) device using the same.
An electroluminescence panel is characterized by high visibility, excellent display performance and a high-speed response. Recently, an organic electroluminescent device (hereinafter referred to also as EL device) using organic compounds is reported (see, for example, relevant document in xe2x80x9cApplied Physics Letters,xe2x80x9d Vol. 51, p913, 1987). This document describes an organic EL device having a structure in which a hole transport layer and an organic light emitting layer are laminated.
The structure of conventional organic EL devices is explained with reference to FIGS. 11 to 14.
An organic EL device 1 of FIG. 11 includes a glass substrate 2, an anode 3 laminated on the glass substrate 2, an organic light emitting layer 4 and a cathode 5. The anode 3 is a transparent electrode.
An organic EL device 1a of FIG. 12 is called an SH-A type device. The organic EL device 1a includes a glass substrate 2, an anode 3 laminated on the glass substrate 2, a hole transport layer 6, an organic light emitting layer 4 and a cathode 5. The above-mentioned report by Tang et al. relates to the organic EL device 1a, which uses a tris (8-quinolinolato) aluminum (hereinafter, referred to also as Alq) for the organic light emitting layer 4. This Alq is an excellent light emitting material having both high luminous efficiency and high electron transporting property. Another type of the organic EL device 1a is a device in which Alq, which forms the organic light emitting layer 4, is doped with a fluorescent dye such as a coumarin derivative or DCM1, etc. (see Journal of Applied Physics, Vol. 65, p3610, 1989). According to the organic EL device 1a, the luminescent colors can be varied and the luminous efficiency can also be improved by selecting appropriate dyes.
An organic EL device 1b of FIG. 13 is called an SH-B type device. The organic EL device 1b includes a glass substrate 2, an anode 3 laminated on the glass substrate 2, an organic light emitting layer 4, an electron transport layer 7 and a cathode 5. For the electron transport layer 7, an oxadiazole derivative, typically 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (hereinafter, referred to also as PBD), is often used. However, the oxadiazole derivative such as PBD etc. tends to be crystallized and is not suitable for practical use.
An organic EL device 1c of FIG. 14 is called a DH type device. The organic EL device 1c includes a glass substrate 2, an anode 3 laminated on the glass substrate 2, a hole transport layer 6, an organic light emitting layer 4, an electron transport layer 7 and a cathode 5.
At present, the SH-A type devices of FIG. 12 or the DH type devices of FIG. 14 are mainly studied and developed. As to the electron transport layer, the hole transport layer and the light emitting layer, new materials suitable for each layer""s function have been developed and studied. In particular, as a hole transporting organic molecule used for the hole transport layer, a large number of materials derived from triphenylamine have been developed. Furthermore, for a fluorescent organic molecule used for the light emitting layer, a fluorescent pigment, a laser dye, etc. can be used.
Furthermore, the cathode 5 is stable and can easily inject electrons when the cathode 5 is made of an alloy of an alkali metal or an alkaline-earth metal having a low work function and a stable metal such as aluminum or silver, etc. According to one report, for example, by controlling a Li concentration contained in the alloy to be such a slight amount as 0.01 wt. % or more and 0.1 wt. % or less, an EL device can exhibit a high luminous efficiency and high stability (see, for example, Publication of Japanese Patent Application No. Hei 5-121172 A). Furthermore, according to another report, a cathode in which a thin film made of a metal having a low work function is formed as an electron injection electrode on the side of an organic compound layer, and a thin film made of a stable metal is formed thereon as a protective electrode. With such a cathode structure, since it is not necessary to control the concentration of an alkaline metal such as Li, etc. having a high reactivity with water, a cathode that easily can be manufactured can be obtained.
Furthermore, an organic EL device in which an electron injection layer is placed on the cathode at the side of the organic layer is also reported. For example, the organic EL device using an alkaline metal compound for the electron injection layer and optimizing the thickness of this electron injection layer is disclosed (see Publication of Japanese Patent Application No. Hei 9-17574A). Furthermore, the relationship between the thickness of these electron injection layers and a dark spot is reported in detail (T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada and M. Tsuchida, IEEE Transactions on Electron Devices, Vol. 44, No. 8, p1245, 1997).
As mentioned above, in the organic EL device, the electron injection layer has been studied as an important factor for determining the luminous efficiency and lifetime.
However, conventional organic EL devices do not have satisfactory luminous efficiency, selectivity of emission wavelength and lifetime.
In particular, when the light emitting layer is formed by a doping method, it is difficult to control the concentration of a dopant. Furthermore, guest materials for emitting red light have a wide range of xcfx80 electron system, so that they tend to be interacted with each other and are easily subjected to concentration quenching.
It is therefore an object of the present invention to provide a organic material suitable for an electroluminescent device and an electroluminescent device using the same.
In order to achieve the above-mentioned object, a first organic material for an EL device of the present invention is a complex compound expressed by the following formula (F1): 
wherein R1 and R2, which can be same or different, respectively denote one selected from the group consisting of halogen, alkyl having 1 to 3 carbon atoms, and a bridging ligand having a nitrogen-containing aromatic ring containing at least two nitrogen atoms, and when R1 or R2 is a bridging ligand having a nitrogen-containing aromatic ring, nitrogen in the nitrogen-containing aromatic ring is a coordinating atom; R3, R4, R5 and R6, which can be same or different, respectively denote one member selected from the group consisting of hydrogen, alkyl, aryl, an aryl derivative, a nitrogen-containing aromatic ring and a derivative of a nitrogen-containing aromatic ring; and M is a central metal. The above-mentioned organic material is suitable for the EL device.
It is preferable in the above-mentioned first organic material for an EL device that the complex compound has a pyrazabole structure.
It is preferable in the above-mentioned organic material for an EL device that the complex compound is 4,4,8,8-tetrakis (1H-pyrazol-1-yl) pyrazabole.
Furthermore, a second organic material for an EL device of the present invention is an organic boron complex compound having an electron accepting phenyl group as a ligand.
It is preferable in the above-mentioned second organic material for an EL device that the boron organic complex compound is expressed by the formula (F2): 
wherein R7 to R9, which can be same or different, respectively denote an electron accepting phenyl group.
Furthermore, an EL device of the present invention includes an anode, a cathode and an organic compound layer placed between the anode and the cathode, and the organic compound layer includes the above-mentioned first organic material for the EL device. Thus, an EL device having an excellent property can be obtained.
It is preferable in the above-mentioned EL device that the organic material has a pyrazabole structure.
It is preferable in the above-mentioned EL device that the organic material is 4, 4, 8, 8-tetrakis (1H-pyrazol-1-yl) pyrazabole.
It is preferable in the above-mentioned EL device that the organic compound layer is a light emitting layer. With such a structure, an EL device having a high luminance can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further includes at least one compound selected from the group consisting of a polycyclic aromatic hydrocarbon compound and a compound including nitrogen-containing aromatic ring having 5 or less carbon atoms. With such a structure, an EL device emitting various colors of light can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further includes a compound having at least one ring structure selected from the group consisting of an anthracene ring and a phenanthrene ring. With such a structure, an EL device emitting various colors of light can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further includes diphenylanthracene or a diphenylanthracene derivative. With such a structure, an EL device emitting various colors of light can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further includes a compound having a ring structure that is the same as the ring structure of at least one member selected from the group consisting of pyrrole, imidazole, pyrazole, triazole, pyridine, pyridazine, pyrazine, pyrimidine, triazine, tetrazine, oxazole and oxadiazole. With such a structure, an EL device emitting various colors of light can be obtained.
It is preferable in the above-mentioned EL device that the peak of the photoluminescense of the compound is 580 nm or more and 680 nm or less. With such a structure, an EL device emitting orange to red light can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further comprises an aromatic-substituted amine or a derivative thereof. With such a structure, an EL device having high luminance and emitting a blue light that has high color purity can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer further comprises an organic compound forming an exciplex with the organic material. With such a structure, an EL device having a high luminance and emitting various luminescent colors can be obtained.
It is preferable in the above-mentioned EL device that the organic compound is pyrene or a pyrene derivative. With such a structure, an EL device having a particularly high luminance can be obtained.
It is preferable that the above-mentioned EL device further includes a second organic compound layer placed between the cathode and the light emitting layer and that the second organic compound layer includes the above-mentioned first organic material for an EL device. With such a structure, an EL device, which is not effected by concentration quenching and which emits various colors of light, can be obtained.
It is preferable that the above-mentioned EL device further includes a hole transport layer between the anode and the light emitting layer. With such a structure, an EL device having a particularly high luminous efficiency can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer is an electron transport layer or an electron injection layer. With such a structure, an EL device having a particularly high luminous efficiency can be obtained.
It is preferable that the above-mentioned EL device further includes a light emitting layer between the organic compound layer and the anode.
It is preferable that the above-mentioned EL device further includes a hole transport layer between the light emitting layer and the anode. With such a structure, an EL device having a particularly high luminous efficiency can be obtained.
It is preferable in the above-mentioned EL device that the organic compound layer is an electron injection layer and the thickness of the organic compound layer is 0.2 nm to 2 nm. With such a structure, the organic compound layer works as an electron injection layer and an EL device having a particularly high luminous efficiency can be obtained.
It is preferable in the above-mentioned EL device that the concentration of the organic material contained in the organic compound layer increases toward the side of the cathode. With such a structure, an EL device having a particularly high luminous efficiency can be obtained.
It is preferable in the above-mentioned EL device that the cathode is made of a metal having a work function of 4.0 eV or more. With such a structure, the cathode is hardly deteriorated, so that an EL device having a long lifetime and high reliability can be obtained.
It is preferable in the above-mentioned EL device that the cathode is made of at least one metal selected from the group consisting of Al, Ag, In and Bi.